In the US 'playbook' is an euphemism for "a set of strategies." It is not childish, and often shows up in serious contexts

And what's the serious context of the racing game screenshot, application development using Flash, YouTube, and Twitter?

It's not only the name (PlayBook = PlayStation in book format) but also the rest of the advertising.
How can anybody believe in its professionalism and readiness for enterprise if RIM highlights toy features all the time?